how to call a SIP phone by its IP-address

The callee has a number of accounts configured on his SIP phone (resp. SIP capable router or whatever (a FRITZ!Box 7390 in my case)).
One of the accounts is 456123 at whatever SIP provider. (This works for all the accounts set up on your SIP phone.)
It is strange, but for this purpose here, it does not matter, which SIP provider this account is tied with.

The DNS name of your SIP phone is xyz.dyndns.org.

You can now call the SIP phone at sip:456123@xyz.dyndns.org.

Of course you yourself have to use a SIP capable phone or software in order to place a call to such an address.

For the FRITZ!Boxes you remove the leading “sip:”, so you only enter 456123@xyz.dyndns.org as a phone number on your 1st phone book of your FRITZ!Box, and of course the entry needs a name. The phone book entry will also get a shortcut assigned (that you can alter, if you want to), and you have to leave a (non-ambiguous) shortcut there, even if you don’t make direct use of it.
You can than call that remote SIP phone by this shortcut.
On a FRITZ!Fon MT-F you can also call that remote SIP phone “directly” from the FRITZ!Box that 1st phone book visible on the FRITZ!Fon MT-F.

I learned tonight, was, that you can (only) call the callee using the “account names” of the SIP accounts, he has with any resp. all SIP providers (apparently) outside.

I actually hope, there is a way to set up an SIP account on the callee’s SIP phone, that is not tied to a SIP provider at all. I don’t see the serious reason for that.
There should be a way, the callee answers on accounts, that he does not necessarily hold with SIP providers “outside”. But I don’t know yet, how to set up such an account on my SIP capable router.
I do actually know, how to set up such an account for the “Local Area Network” of my router, but I don’t know yet, how to do this for the “Wide Are Network” side of my router.

To Be Continued …